1. Field of the Invention
This invention relates to a grounding device for preventing electrolytic corrosion in the bearings of rotary electric machines. More specifically, the invention relates to a grounding device for preventing electrolytic corrosion in the bearings of rotary electric machines, which inhibits the electric current from flowing into the bearings at the time when the rotary electric machine is started where the grease in the bearings exhibits a small resistance to the shaft voltage that is generated in the rotary electric machines.
2. Description of the Prior Art
FIG. 1 is a diagram which schematically illustrates a conventional rotary electric machine. In FIG. 1, a rotor 2 is disposed in a grounded stator 1. A shaft 2a of the rotor 2 is supported by bearings 3 which contain a lubricating agent 3a. Grease is generally used as a lubricating agent 3a. As is well known, the grease exhibits an electric resistance which is large when the bearings are rotating, and exhibits a small resistance when the bearings are at rest or are rotating at speeds close to rest. A voltage is generated in the shaft 2a due to a variety of reasons. For instance, a voltage is electrostatically induced in the shaft 2a when a high-frequency current flows into the coil or bar 2b of the rotor 2. A shaft voltage is generated at the moment a starting voltage is applied to the rotary electric machine. At that moment, the rotary electric machine still does not rotate due to inertia, and the grease 3a in the bearing 3 exhibits a small resistance, thereby allowing a large electric current i to flow through the shaft 2a, bearing 3 and stator 1 as shown in FIG. 1, giving rise to the occurrence of electrolytic corrosion in the bearing 3. To prevent the electric current from flowing into the bearing 3, therefore, a brush 5 had heretofore been attached to an electrically conductive end cover 4 by a spring 6 as shown in FIG. 2. According to this construction, the shaft 2a is pushed by the brush 5 to short-circuit the bearing 3. With the thus constructed conventional device, however, the brush 5 is subject to be worn out, and cumbersome maintenance is required, i.e., the brush 5 must be checked and renewed frequently.